Conventionally, so-called Ziegler-Natta catalysts which generally consist of a titanium or vanadium compound as a primary catalyst component and an alkylaluminium compound as a cocatalyst component have been usually employed for preparing copolymers of ethylene with α-olefins. Though a Ziegler-Natta catalytic system exhibits high activity on ethylene homo-polymerization, the catalytic system is disadvantageous (due to its irregular catalyst activation point) in that a large amount of α-olefin comonomer should be used for preparing elastomeric copolymer of ethylene with α-olefin, having the density of 0.910 or less, because of its poor reactivity of copolymerization with higher α-olefin. In this situation, activity of the catalyst becomes lower. Further, the copolymers prepared by using such catalyst show very irregular distribution of composition and broad molecular weight distribution, thereby being unsuitable as elastomer.
Recently, metallocene catalytic systems consisting of a metallocene compound of Group 4 transition metal in the Periodic Table of Elements, such as titanium, zirconium and hafnium, and methyl aluminoxane as a cocatalyst have been developed. Since the metallocene catalytic system is a homogeneous catalyst having a mono-modal catalyst activation point, it can provide copolymers of ethylene and α-olefin, having narrow molecular weight distribution and homogenous composition distribution as compared to conventional Ziegler-Natta catalyst.
For example, European Patent Publication Nos. 320,762 and 3,726,325; Japanese Patent Laid-Open Nos. Sho 63-092621, Hei 02-84405 and Hei 03-2347 reported that ethylene and α-olefin can be copolymerized with high activity by activating the metallocene compounds such as Cp2TiCl2, Cp2ZrCl2, Cp2ZrMeCl, Cp2ZrMe2, ethylene(IndH4)2ZrCl2 by using methyl aluminoxane as cocatalyst, to provide copolymer of ethylene with α-olefin having the molecular weight distribution (Mw/Mn) in the range from 1.5 to 2.0. However, the catalytic system is disadvantageous, when it is applied to solution polymerization carried out at a high temperature of 80° C. or higher, in that a large amount of higher α-olefin has to be employed (just as in the case of using Ziegler-Natta catalyst) in order to prepare elastomers having the density of 0.910 or less, due to steric hindrance of the catalyst itself. The process is not suitable for preparing high molecular weight polymer with weight average molecular weight (Mw) of not less than 30,000 because β-dehydrogenation preferentially occurs in this situation.
In the meanwhile, disclosed were so-called geo-restrictive non-metallocene type catalysts, wherein the transition metals are linked in the form of a ring, as catalysts for preparing high molecular weight polymers with high catalytic activity in copolymerization of ethylene with α-olefin under the condition of solution polymerization. European Patent Nos. 0416815 and 0420436 suggested the examples wherein amide groups are linked in the form of a ring to one cyclopentadiene ligand, while European Patent No. 0842939 showed exemplary catalysts wherein phenolic ligands (as electron donors) are linked to cyclopentadiene ligand in the form of a ring. The geo-restrictive catalyst could provide noticeable enhancement in reactivity with higher α-olefins, due to reduced steric hindrance of the catalyst itself. However, there are many difficulties to realize an economic and effective process for preparing copolymers of ethylene with α-olefin, having the density of not more than 0.910, since the synthetic process for the catalyst is complicated, and the yield of the procedure of ring formation between the ligands and the transition metal compounds is very low.
On the other hand, examples of non-metallocene catalysts that are not geo-restrictive can be found in U.S. Pat. No. 6,329,478 and Korean Patent Laid-Open No. 2001-0074722. It is found that the catalyst of single activation point, which employs at least one phosphinimine compound(s) as a ligand(s), showed high ethylene conversion in the copolymerization of ethylene with α-olefin under the condition of solution polymerization at a high temperature of 140° C. or more. However those catalysts showed low reactivity with higher α-olefin, being similar to the metallocene catalyst, so that it is not suitable for preparing elastomers of ethylene with higher α-olefin.